With high heat resistance and good mechanical properties, aromatic polyamides have various potential uses in the form of fiber, film, and aramid paper. Para-oriented aromatic polyamides, in particular, are higher in rigidity, strength, and mechanical properties than other polymers, and widely known as the material for high-strength fibers such as "Kevlar" . When used as material for film, they can produce very thin products, with potential ones including printer ink ribbon, magnetic tape, and capacitors. Aromatic polyamides with better surface properties including running performance and abrasiveness are now demanded as more rapid processes for aromatic polyamide processing are adopted such as for producing coated-type or metal thin magnetic layers in magnetic recording medium and forming image transfer layers for thermal recording. As magnetic recording medium products have been decreased in size and increased in recording density, formation of uniform, fine protrusions over the surface of aromatic polyamide film, which acts as the base film, is required to ensure a good balance among the output characteristics, running properties, and durability of the resulting magnetic recording medium. In the recently-developed magnetic recording medium consisting of a metal thin magnetic layer, in particular, the surface characteristics of the base film can have large influence on the surface characteristics of the magnetic layer, and this has increased the demands for such good base film materials.
Compared to aromatic polyimides and polyesters such as polyethylene terephthalate and polyethylene naphtha late that have been conventionally used as base film of magnetic recording medium, aromatic polyamide film can be very thin due to its high rigidity and therefore is very suitable material for high-capacity magnetic recording medium. The major difference from the polyester film which is produced by melt casting process is that aromatic polyamide film is produced by solution casting process. Compared to aromatic polyimide film which is produced by solution casting process, aromatic polyamide film is also very different in the pH variations during the film forming process and in that polymerizing solutions contains inorganic salts in many cases, requiring particular care for the desolvation step in the film forming process. Thus, special techniques suitable for aromatic polyamides have to be developed to solve these problems.
The use of inorganic particles contained in aromatic polyamide film is known as a technique to produce fine protrusions over the surface (e.g., Japanese Laid-Open Patent Applications (Kokai) SH060-127523 and SHO60-201914). Another known example is a aromatic polyamide film consisting of a base layer laminated with a thin layer containing particles for producing surface protrusions (eg., Japanese Laid-Open Patent Applications (Kokai) HEI3-119512).
However, above-mentioned conventional aromatic polyamide films containing (inorganic or other) particles as major component of the surface protrusions have three serious basic problems as described below.
First, if inorganic or other foreign particles with different properties are added to aromatic polyamide to form surface protrusions, the affinity with the inorganic material may be reduced due to the strong intermolecular force of aromatic polyamide, which will cause removal of protrusions, and the film surface may become liable to scraping or other damage that will decrease the abrasion resistance of the film surface.
Second, finer protrusions have been increasingly required in recent years, as described above, and film containing smaller- diameter particles is tend to be used to meet this requirement. However, as the diameter of particles contained decreases, the repulsion among particles decreases to cause their coagulation, resulting in a decreased uniformity of the distribution of protrusions and an increased number of large protrusions. This is an essential problems arising from then mixing of aromatic polyamide with particles of a foreign nature, and this problem cannot be avoided completely when solid particles, organic or inorganic, are used. Furthermore, aromatic polyamide during the process for preparing the polymer solution is liable to suffer large changes in pH of the solution to accelerate the coagulation of particles. Thus, the use of aromatic polyamide is disadvantageous regarding this particle coagulation.
Third, during such a processing process as state above or during the use of a final product, aromatic polyamide film may often travels along guide rolls or guide pins, but the protrusions consisting of particles contained are generally so high in hardness that the surface of the guide will be scraped to produce dust which in turn will fall on the film. This problem will be particularly serious if the guide is made of plastics or if the film travels along it rapidly and repeatedly.
Japanese Laid-Open Application (Kokai) HEI 8-255332 has disclosed a magnetic recording medium consisting of a magnetic layer formed over a non-magnetic support with a prescribed surface roughness. The material for the support actually disclosed, however, is polyester film, instead of aromatic polyamide film. Further, no specific methods for preparing the surface of the support are virtually disclosed, and the only description regarding surface preparation is that it is done by adjusting the film formation conditions for surface smoothing and by adjusting the size and density of the particles added, which cannot work effectively enough to solve the above-mentioned problems.
Japanese Laid-Open Application (Kokai) HEI 3-237135 has disclosed heat resistant film consisting of aromatic polyamide and soluble resin with the content of the soluble resin being in the range of 10 wt % to 95 wt %. This, however, aims at enhancement of the chemical properties and cost reduction, and there is no description or suggestion regarding a technical idea to produce a surface with a fine texture. Further, the content of the soluble resin is large compared to aromatic polyamide, reducing the good mechanical properties, the high Young's modulus in particular, characteristic of aromatic polyamide. For instance, the one with the highest Young's modulus described in Examples in the Application is only 6.2 GPa (Example 2). Similar inventions are disclosed in Japanese Laid-Open Applications (Kokai) HEI 3-286680, HEI 3-227290, HEI 4-117433, and HEI 4-27110, but they have similar problems.
Japanese Laid-Open Application (Kokai) HEI 7-44857 has disclosed a film consisting of a magnetic surface with a special fine texture that is produced by blending two aromatic polyamide components, one being the major component and the other being the minor component different from the former and lower in solubility, followed by a molding process in which the minor component aromatic polyamide is deposited on the surface. However, aromatic polyamides, particularly para-oriented ones, generally are not high in solubility, and if an aromatic polyamide is blended with another which is still lower in solubility, it will be difficult for them to mix completely to form a homogeneous polymer solution, even though complete dissolution seems apparent. A study by the present inventors has shown that film produced from such a polymer solution contains many rough protrusions with a height of 150 nm or more, despite the coexistence of fine protrusions, and that the surface tends to have crater-like dents that cause surface roughness. It has also been shown that if two aromatic polyamide components, major Ad and minor, that are very similar in structure and consequently nearly the same in solubility are blended, they seem to mix with each other completely and only a few protrusions are formed. Therefore, it is difficult for the technique disclosed in the Application to produce uniformly-distributed fine protrusions.
Japanese Laid-Open Application (Kokai) SHO 59-122547 has an disclosed a polyimide resin composition with good slip characteristics that is produced by adding a resin containing a sulfone group to an aromatic polyimide up to 0.01 wt % to 10 wt %. However, aromatic polyimide and aromatic polyamide show very different characteristics when, for instance, in the form of film, so the technique belongs to a different technical field.
Japanese Laid-Open Application (Kokai) HEI 4-8763, which relates to methods for producing aromatic polyamide/polyethersulfone composition, has disclosed the technique of adding polyethersulfone prior to the completion of the aromatic polyamide polymerization. There is no description, however, regarding the properties of resulting moldings (e.g., film), let alone methods to design or produce a surface with a specific texture, and no example in the Application refers to a molding process such as for film formation.
The primary object of the present invention is to provide aromatic polyamide resin moldings with virtually uniform, fine surface protrusions that are difficult to destroy because of their very high affinity with aromatic polyamide compared to those formed in conventional methods, which are realized, virtually not by addition of particles, but by microscopic-level separation, during the molding process, of the aromatic polyamide from the dissimilar polymer which, in the solution, is compatible with the former or in a dissolved state.